Re: [bitcoin-dev] Introducing a POW through a soft-fork

From: Devrandom <c1.bitcoin@niftybox•net>
To: Eric Voskuil <eric@voskuil•org>,
	 Bitcoin Protocol Discussion
	<bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Introducing a POW through a soft-fork
Date: Tue, 07 Nov 2017 04:38:51 +0000	[thread overview]
Message-ID: <CAB0O3SX-m1Uw8Ga-ddzyU6oYdM2QRXn2OetgPmPBT+-5wGmjKQ@mail.gmail.com> (raw)
In-Reply-To: <61253DDB-A045-4346-A39C-F5C4E07396C7@voskuil.org>

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A hard-fork is a situation where non-upgraded nodes reject a block mined
and relayed by upgraded nodes.  This creates a fork that cannot heal
regardless of what follows.

This proposal is not a hard-fork, because the non-upgraded node *will heal*
if the attack has less than 1/2 of the original-POW power in the long term.

The cost of such an attack is the cost of a normal "51%" attack, multiplied
by the fractional weight of the original POW (e.g. 0.75 or 0.5).

So rather than saying this is a hard-fork, I would say that this is a
soft-fork with reduced security for non-upgraded nodes. I would also say
that the reduction in security is proportional to the reduction in weight
of the original POW at the time of attack.

As mentioned before, the original-POW weight starts at 1.0 and is reduced
over a long period of time.  I would set up the transition curve so that
all nodes upgrade by the time the weight is, say, 0.75.  In reality, nodes
protecting high economic value would upgrade early.

On Mon, Nov 6, 2017 at 3:55 PM Eric Voskuil via bitcoin-dev <
bitcoin-dev@lists•linuxfoundation.org> wrote:

> If a block that would be discarded under previous rules becomes accepted
> after a rule addition, there is no reason to not simply call the new rule a
> hard fork. IOW it's perfectly rational to consider a weaker block as
> "invalid" relative to the strong chain. As such I don't see any reason to
> qualify the term, it's a hard fork. But Peter's observation (the specific
> behavior) is ultimately what matters.
>
> e
>
> On Nov 6, 2017, at 12:30, Paul Sztorc via bitcoin-dev <
> bitcoin-dev@lists•linuxfoundation.org> wrote:
>
> +1 to all of Peter Todd's comments
>
> On Nov 6, 2017 11:50 AM, "Peter Todd via bitcoin-dev" <
> bitcoin-dev@lists•linuxfoundation.org> wrote:
>
>> On Wed, Nov 01, 2017 at 05:48:27AM +0000, Devrandom via bitcoin-dev wrote:
>>
>> Some quick thoughts...
>>
>> > Hi all,
>> >
>> > Feedback is welcome on the draft below.  In particular, I want to see if
>> > there is interest in further development of the idea and also
>> interested in
>> > any attack vectors or undesirable dynamics.
>> >
>> > (Formatted version available here:
>> > https://github.com/devrandom/btc-papers/blob/master/aux-pow.md )
>> >
>> > # Soft-fork Introduction of a New POW
>>
>> First of all, I don't think you can really call this a soft-fork; I'd
>> call it a
>> "pseudo-soft-fork"
>>
>> My reasoning being that after implementation, a chain with less total
>> work than
>> the main chain - but more total SHA256^2 work than the main chain - might
>> be
>> followed by non-supporting clients. It's got some properties of a
>> soft-fork,
>> but it's security model is definitely different.
>>
>> > ### Aux POW intermediate block
>> >
>> > Auxiliary POW blocks are introduced between normal blocks - i.e. the
>> chain
>> > alternates between the two POWs.
>> > Each aux-POW block points to the previous normal block and contains
>> > transactions just like a normal block.
>> > Each normal block points to the previous aux-POW block and must contain
>> all
>> > transactions from the aux-POW block.
>>
>> Note how you're basically proposing for the block interval to be
>> decreased,
>> which has security implications due to increased orphan rates.
>>
>> > ### Heaviest chain rule change
>> >
>> > This is a semi-hard change, because non-upgraded nodes can get on the
>> wrong
>> > chain in case of attack.  However,
>>
>> Exactly! Not really a soft-fork.
>>
>> --
>> https://petertodd.org 'peter'[:-1]@petertodd.org
>>
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